The invention relates to a wind turbine blade comprising a first and a second electric conductor which extend in the longitudinal direction of said blade, one or more electric heating elements for heating the surface of said blade and being connected to the first and the second conductor.
Wind turbines are often exposed to damaging effects caused by the climatic conditions and the forces of nature. Generally speaking, the invention relates to protection of wind turbines and to be more precise it relates to protection of the wind turbine blades against damaging or undesired effects caused by icing on the surface of the blades and by strokes of lightning.
Icing of aircraft wings is a well-known and severe problem for several reasons. The icing can change the aerodynamic profile of the wing and consequently damage the lift capacity of the wing. Moreover, pieces of ice can break off during the aviation and thereby damage the propellers or the turbine blades. Most places the problem of icing of aircraft wings has been solved by the aircraft being subjected to a spraying with specific chemicals prior to the take-off.
The icing of wind turbine blades has also several negative consequences. The icing can also change the wing profile and consequently the aerodynamic properties of said wing, which has a negative effect on the efficiency. In addition, a non-uniform icing on wind turbine blades can involve a violent asymmetric load on the wind turbine structure, which can result in a breakdown. Finally, pieces of ice shed off the wings present a severe risk to for instance buildings and persons being near the wind turbine. During recent years this problem has been increased because more and more wind turbines are placed in locations suffering from a high risk of icing.
Icing on wind turbine blades can be removed by heating the surface of the blade, and the latter can be carried out in several ways. DE 196 21 485 discloses a wind turbine blade which is internally heated by means of hot air so as to provide a deicing. WO 98/53200 discloses a wind turbine blade which can be deiced by means of heating elements comprising electrically conducting fibres. The heating elements can be arranged on the outside of or embedded in the wind turbine blade. A heating of the surfaces results in a melting of the interface bond between the surface of the wind turbine blade and the ice coating in such a manner that the latter is xe2x80x9cshed offxe2x80x9d as flakes.
Modern wind turbines are often very large structures towering above the landscape, and many wind turbines are subjected to strokes of lightning every year. A stroke of lightning can involve extremely strong currents of the magnitude 10 to 200 kA within a very short period. Damages caused by strokes of lightning vary from short interruptions of the production due to for instance blown fuses, to severe damages where one or more of the blades are damaged, which in turn can cause damages on the entire structure. During recent years systems for protection of wind turbines against strokes of lightning have accordingly been developed.
WO 96/07825 discloses a lightning protection system for wind turbine blades, where the blade tip has been provided with a so-called lightning receptor of electrically conducting material. This lightning receptor can xe2x80x9ccatchxe2x80x9d a stroke of lightning and lead the lightning current downwards through a lightning down conductor which extends in the longitudinal direction of the blade and which is earthed through the wind turbine hub. This system proved to provide a particularly efficient protection.
The object of the invention is to provide a device which in a simple, inexpensive and reliable manner protects a wind turbine blade against both icing and strokes of lighting.
The wind turbine blade according to the invention is characterised in that said wind turbine blade is provided with a lightning conductor system known per se and comprising a lightning receptor at the tip of the wind turbine blade and a third electric conductor electrically connected to said lightning receptor, and which is earthed at the blade root, and that the first and the second conductor are connected through spark gaps adjacent the lightning receptor to said lightning receptor and are earthed through spark gaps at the blade root. As a result a wind turbine blade is obtained which can be deiced, and where the blade and the heating elements are efficiently protected against strokes of lightning, the lightning current primarily being carried to the ground through the third conductor and at formation of electric arcs in the spark gaps secondarily being carried to ground through the first and the second conductor. The potential difference across the heating elements is always zero during the stroke of lightning and thus no damaging strong currents pass through said heating elements.
According to a preferred embodiment of the invention the first conductor adjacent the blade root is connected through an overvoltage cutout to a first current phase, and the second conductor adjacent said blade root is connected to a second current phase through an overvoltage cutout. As a result, an efficient protection of the power supply is obtained because the overvoltage cutouts interrupt the electric connection between the power supply and the first and the second conductor, respectively, in case of strokes of lightning.
According to an advantageous embodiment of the invention the heating elements are placed externally on the surface of the blade. As a result, it is not necessary to change the blade structure because the strength of said blade is not reduced by such a positioning.
According to another embodiment of the invention, the heating elements are laminated into the blade material, whereby the surface properties of the wind turbine blade and consequently the aerodynamic properties of said blade remain unaffected.
The heating elements are preferably made of a metal foil with the result that a uniform heating of the surface of the wind turbine blade is obtained. Such a metal foil can also be glued onto the surface of the wind turbine blade without considerably changing the aerodynamic properties thereof.
According to an embodiment, the first and the second conductor may be arranged in the wall of the wind turbine blade and the third conductor may be inserted in a cavity in said blade. As a result a small interspaced is ensured between the heating elements and the first and the second conductor, respectively.
The heating elements are preferably arranged at the front rim of the blade because the icing often starts here during the operation of the wind turbine.
The wind turbine blade according to the invention may comprise a pivotal tip, where at least a portion of the first and the second conductor extends into said pivotal tip and is connected through resilient contact members to the portion of the same conductor extending in the remaining blade portion, and where spark gaps can be provided parallel to each contact member between the two portions of the As a result, an electric connection is obtained between the portions of the first and the second conductor extending in the tip and the remaining blade, respectively, while the contact members are simultaneously protected against lightning current and consequently the risk of a welding together because said lightning current passes through an electric arc in the spark gaps. As a result a simple and efficient protection of the wind turbine blades with a pivotal tip against icing and strokes of lightning is obtained.